1. Field of the Invention
The present invention pertains to charge pump power supplies for generating bipolar output voltages greater in magnitude than a single unipolar input voltage. More particularly, the present invention pertains to the integration of such a circuit on a single piece of semiconductor substrate material. Further, the invention pertains to other circuitry integrable on a single piece of semiconductor substrate material along with such a power supply circuit.
2. Prior Art
Discrete component voltage doubler and voltage inverter circuits are well known in the art. Such circuits are used in many electronic systems which require a multiplicity of DC voltages for operation. More recently, in the context of digital circuits and systems, it has become common to employ a single five volt unipolar voltage supply to power digital circuitry in modern data processing systems. For example, semiconductor microprocessors, memories, and logic all commonly operate from a single five volt power supply. There are however, certain interface circuits and other special purpose circuits which require voltages other than five volts. More particularly, some circuits require voltages in the ranges of from five to fifteen volts. Additionally, requirements often exist for bipolar power supply voltages so that voltage power requirements of plus or minus 15 volts and plus or minus 12 volts are commonly encountered, for example in RS-232 communication loops.
For these communicaton circuits and other applications, bipolar DC power requirements are low when compared to the digital circuitry power requirements. In fact, it is common to encounter five volt unipolar power supplies for driving digital logic rated in tens or hundreds of watts whereas interface and other power requirements may be as low as tens or hundreds of milliwatts.
It is therefore often desirable to generate locally the various non-primary voltage sources, i.e., the bipolar voltage sources, if the power requirements are not high and if it can be done economically and with relatively high electrical power conversion efficiencies.
For an example, a minicomputer may have a 100 watt 5 volt power supply which supplies all of the requirements for a multiplicity of printed circuit boards holding logic integrated circuits. On one of those integrated circuit boards, there will often be an RS-232 digital interface circuit requiring a plus or minus 10 or plus or minus 15 volt power supply. This interface circuit may consume 50 milliwatts of power. Instead of generating the plus and minus 15 volt power supply from the main power supply and then bussing these voltages to the boards which require them, it is often more economical to generate these two voltages from the bussed five volt power supply locally on whatever board needs other voltages. However, generating such voltages by the use of discrete components is often disadvantageous because the additional components required to generate such voltages take up a relatively large amount of circuit board real estate, and often are power inefficient, i.e., heat producing.
The industry has recently turned its attention to attempts to furnish auxillary power supplies of the nature herein described on a single semiconductor substrate. Such circuits have the obvious advantages of space saving, assembly labor savings, and relatively lower power dissipation. A form of such circuits known as charge pumps have been used in semiconductor memory chips to produce a crude back bias supply and for supplying the higher voltages needed to program such memory devices. Charge pump circuits have been used in the inverting mode to produce voltage polarities opposite to that of the supply voltage from which they are generated. An example of such a circuit is found in the product designated ICL 7660, a power supply circuit manufactured by the assignee of the present invention.
The efforts to design and implement a bipolar charge pump integrated circuit have met with several obstacles which result from the inherent nature of the integration process and the fabrication process which are used to manufacture these devices. It is well known to those in the art that when MOS or CMOS circuits are integrated onto a single semiconductor substrate, the chip layout geometry and architecture inherently produce parasitic junction devices. These devices include junction biodes, bipolar transistors, and PNPN four layer diode devices, similar to silicon controlled rectifier (SCR) devices.
The existence of these parasitic devices has created difficulties in the design and fabrication of dual polarity charge pump power supply circuits. When forward biased, the aforementioned four layer diode device will cause a CMOS circuit to experience a phenomenon known as latch-up. Latch-up is a phenomenon common to CMOS circuts whereby the circuit can be triggered into a low impedance conducting state by forward biasing an inherent four layer diode device in the circuit. This four layer diode may be triggered by various means into a low voltage, low impedance state. When this occurs, operation of the circuit is inhibited and possible damage may occur to the circuit if there is no inherent current limiting designed into the circuit.
Another problem inherent in the design of dual polarity charge pump inverter circuits is the difficulty of assuring correct start-up of the circuit. The conditions existing in the semiconductor material at the time of start-up may randomly produce states which prevent such a circuit from ever starting up to produce the desired output voltages. In the past, elaborate systems and considerable extra circuitry has been designed into such circuits in an attempt to avoid this problem.